Where is it located? This usually provides the answer to the third and most important question:

What are the most likely causative organisms?

What are the likely antimicrobial susceptibilities of these organisms?

Initial management is empiric; to answer questions 3 and 4, one should use the following: (1) information from published studies of this infection; and (2) information from your specific unit.

Microbiologic information should be obtained from the patient to provide directed therapy. The most important principle in obtaining microbiological information is that specimens should be obtained from the site or presumed site of infection whenever possible. Blood cultures give information only about organisms in the blood.

The second decision to be made:

Given the answers to the first set of questions, what is the optimal agent to use?

This must take into account:

The necessary antimicrobial spectrum of the agent(s).

Pharmacokinetics.

Will it reach the site of the infection?

Adverse effects.

Drug-drug interactions.

Route of administration.

Cost (use the cheapest drug, other factors being equal).

The overriding principle is to use the narrowest spectrum agent possible for the clinical circumstance.

The susceptibilities of different bacteria to different antimicrobial agents are fairly predictable. However these susceptibilities vary over time and place. Therefore, it is important to know the susceptibilities of specific organisms in your hospital and unit.

Table I indicates commonly encountered/important microbial agents in the intensive care unit, and their usual susceptibilities.

Table II lists the organisms causing different types of infections at different sites in different types of hosts and suggested empiric therapy (i.e. the initial therapy based on the clinical and epidemiological information, prior to the availability of microbiological information). Once or if a specific agent is identified, therapy should be tailored accordingly.

Microbial causes of infections according to site, and type of host, and recommended empiric antimicrobial therapy.

Dosages, dosing intervals, and routes of administration of antimicrobial agents, by categories, for children after the newborn period and adults.

Table IV lists the major adverse effects of antimicrobial agents.

Table IV:

Major adverse effects of antimicrobial agentsBecause most agents can cause allergic reactions, vomiting or diarrhea, some abdominal discomfort, and drug-drug interactions, these are mentioned only in cases in which they are particularly prominent and/or important. Their relative frequency is difficult to determine. Almost all antibacterial agents can cause Clostridium difficile colitis, and other overgrowth conditions, such as candidiasis, and many can cause phlebitis when administered intravenously. Many can cause drug fever. In addition, the use of antimicrobial agents in an individual patient contributes towards the emergence of antimicrobial resistance in the community.